The 3rd Generation Partnership Project (3GPP) is discussing application of the Hybrid Automatic Repeat Request (HARQ) scheme to a channel for high-speed transmission of packet data on the uplink. Such channel is known as an enhanced-uplink dedicated channel (E-DCH).
In an application of the HARQ system to the uplink, a network (for example, node-B) transmits, in response to a packet sent from a user equipment (UE), a negative acknowledgement (NACK) signal to the UE when it is determined, based on the results obtained after decoding the packet, that the packet is erroneously decoded, and transmits an acknowledgement (ACK) signal to the UE when it is determined that the packet is successfully decoded. The ACK/NACK signal is transmitted through a 1-bit ACK/NACK channel.
In response to reception of the NACK signal, the UE re-transmits the previously-transmitted packet. The node-B combines decoding information about the re-transmitted packet with decoding information about the previously-transmitted packet, using diverse transmission methods to enhance the reception performance for the re-transmitted packet. In this case, the ACK/NACK signal is a 1-bit signal which is transmitted on the downlink. For such an ACK/NACK signal, no channel coding is provided to protect it from interference or other adverse communication conditions. For this reason, it is necessary to use very high transmission power to enable the UE to successfully receive the ACK/NACK signal.
In the HARQ system, the NACK signal must have a higher transmission success rate than that of the ACK signal. When the UE erroneously receives an ACK signal as a NACK signal, the UE re-transmits the packet, which was already received by the node-B. Accordingly, no harm occurs other than re-transmitting the same data. However, when the UE erroneously receives a NACK signal as an ACK signal, the UE erroneously determines that the associated packet data was successfully transmitted to the node-B, and then transmits a subsequent packet. As a result, the erroneously-decoded packet can no longer be recovered at the physical layers of the node-B. The recovery of the erroneous packet must be conducted at an upper layer. As a result, the packet transmission delay is increased.
Furthermore, when the UE is in fast transit, or is in soft handover with a plurality of node-Bs, the link state between the UE and each node-B may be very weak. In this case, the ACK/NACK transmission power (in particular, the NACK transmission power) required to obtain a satisfactory ACK/NACK reception performance may be impractically high.
Accordingly, it is necessary to appropriately set the downlink ACK/NACK transmission power, taking into consideration all of the above-mentioned possibilities.
There remains therefore a need for an improved means for ACK/NACK communication in a wireless systems using the enhanced uplink dedicated channel that solves these and related problems.